Automated fluid injection devices, particularly automated needle syringes, have gained wide acceptance by industry and by the scientific and medical communities. This is because these devices are generally capable of dispensing very small, accurately measured quantities of fluid specimens on the order of a few microliters, generally a fractional part of a microliter up to about 50 microliters with high accuracy and precision. Moreover, the advantages offered by modern data gathering techniques, and consequent reduction in operating manpower without loss in accuracy make these devices particularly useful in modern industrial establishments.
Typically, in the operation of an automated fluid injection device, septum covered vials are charged with a fluid specimen and transported in seratim via a magazine to a station adjacent a probe assembly, a needle of the probe assembly is projected through the septum of a vial and employed as a conduit to convey a portion of the fluid specimen to the barrel of the syringe. The circuit through which the specimen is conducted, and barrel and needle of the syringe are cleaned, purged and a quantity of the fluid specimen is measured out and injected via the needle end of the syringe into the inlet of an analytical instrument, e.g., a G.C. or mass spectrometer.
Time, pressure and the relationship between time and pressure are critical parameters in designing instruments of this type. Pressure is the driving force for movement of a fluid specimen from a vial to the barrel of the syringe and the rate of movement of the fluid specimen is directly related to the viscosity of the fluid specimen. The higher the viscosity of the fluid specimen at a given pressure the slower its rate of movement. Conversely, the lower the viscosity of the fluid specimen at a given pressure the greater the rate of movement of the fluid specimen. Consequently, whereas automatic fluid injectors have served admirably well, they nonetheless prove unreliable, and even fail in handling highly volatile or highly viscous fluid specimens. For example, the accuracy and precision of these instruments in handling fluid specimens of high volatility, e.g., hexane which measures about 0.3 centipoises, and fluid specimens of low volatility, e.g., propylene glycol which measures about 50.0 centipoises, at best leaves much to be desired in terms of accuracy and precision, and at worse fails miserably. In handling the more volatile specimens, the specimens on delivery to the barrel and needle portion of the fluid injection device thus flow too freely and thus drain too rapidly from the dispensing end of the needle. Thus, the barrel and needle are not completely filled. In handling the more viscous specimens, the fluid flows so slowly from the pick up station to the barrel and needle portion of the fluid injection device that the barrel and needle are not adequately filled. For these reasons, it becomes virtually impossible to handle at one time a set of vials loaded with fluid specimens of a wide range of viscosities, and virtually impossible to adjust the cycle length of the cleaning, purging and filling operation over the range of viscosities necessary for many fluid sampling operations. Highly viscous fluids are per se difficult to handle in conventional fluid injection devices, and many viscous fluids simply cannot be handled by conventional fluid injection devices. There is thus a need for improved automatic fluid injectors capable of loading and injecting, with accuracy and precision, highly volatile fluid specimens and highly viscous fluid specimens, particularly the latter, and more particularly fluid specimens of a set having a range of differing viscosities.